1. Technical Field of the Invention
The present invention relates to the preparation of mixed oxides based on vanadium, on antimony and, optionally, on tin and/or on titanium and/or on iron and/or on other metals deposited onto a support therefor and to the use of such supported mixed oxides as catalysts for the ammoxidation of alkanes.
2. Description of the Prior Art
Certain mixed oxides of vanadium and antimony, or of vanadium, antimony and other metals, are known compounds which have been described, among many other mixed oxides, in FR-A-2,072,334.
U.S. Pat. No. 5,008,427 describes the ammoxidation of propane or butane in the presence of a catalyst which can, in particular, comprise oxides of vanadium, of antimony and of iron or of titanium or of chromium or of gallium or of tin and, optionally, of other metals. These catalysts have the essential characteristic of having been calcined at a temperature equal to or greater than 780.degree. C.
Similarly, EP-A-0,558,424 describes the ammoxidation of alkanes in the presence of a mixed oxide catalyst of vanadium, of antimony, of iron and/or of gallium and/or of indium. These mixed oxides are prepared by mixing aqueous suspensions of compounds of the various metals, heating with stirring and then evaporating the water, drying and calcination.
The aforesaid patent literature provides no specific advice regarding the reactor technology associated with the catalysts for the ammoxidation of alkanes which they describe.
However, for the type of reaction represented by the ammoxidation of alkanes, it transpires that employing a fluidized bed or moving bed reactor is more advantageous. Due to the high exothermicity of the reactions involved in the ammoxidation of alkanes, the possibility of using one or a number of stationary bed reactors (multitubular reactors) on an industrial scale does not appear very realistic, indeed excluded, in particular if it is desired to achieve high productivities. In fact, it would be necessary, in order to dissipate the heat evolved, to use a multitubillar reactor of very large size or an entire battery of multitubular reactors, which is not feasible from an economic standpoint.
In comparison with the stationary bed reactor, fluidized or moving bed reactors have a greater ability to dissipate the heat evolved and appear, for this reason, better suited for the ammoxidation reaction of alkanes. This better ability to transfer heat would permit enhancing productivity by increasing the alkane content in the feedstream mixture. Such an operation is not possible in the case of a stationary bed reactor because the limitations with respect to heat transfer (which can be detected by the existence of a hot spot in the reactor) dictate the use of relatively low alkane contents, to avoid any danger of explosiveness or flammability of the gas mixture.